The use of monoclonal antibodies (mABs) to deliver anticancer drugs directly to tumor cells has attracted a great deal of focus in recent years. Two new antibody-drug conjugates have been approved by the FDA for the treatment of cancer. Adcetris® (brentuximab vedotin) is a CD30-directed antibody-drug conjugate (ADC) indicated for the treatment of relapsed or refractory Hodgkin's lymphoma and systemic anaplastic large cell lymphoma (ALCL). Kadcyla® (ado-trastuzumab emtansine) is a new therapy approved for patients with HER2-positive, late-stage (metastatic) breast cancer. To obtain both potent anti-tumor activity and an acceptable therapeutic index in an ADC, several aspects of design may be optimized. Particularly, it is well known that the chemical structure of the linker can have significant impact on both the efficacy and the safety of ADC (Ducry & Stump, Bioconjugate Chem, 2010, 21, 5-13). Choosing the right linker influences proper drug delivery to the intended cellular compartment of cancer cells. Traditionally, options for conjugating drugs (also sometimes called toxins) containing tertiary amines or pyridine functional groups have been limited mainly to attaching these to other functional groups in the molecule. Therefore, it is desirable to develop methods for linking amine or pyridine containing toxins by forming a quaternary salt. Provided herein are different types of quaternary salt containing ADCs that can be cleaved in tumor cells.